The need for fast transmission of signals in optical fibers is general increasing. Whereas today's optical interconnects are based on optical NRZ (non return to zero) modulation formats with two optical power levels, future interconnects might be based on other modulation schemes, multi-level optical links being one proposal, and several methods of converting electrical signals to multilevel signals have been suggested.
U.S. Pat. No. 8,380,085 (NEC LABORATORIES AMERICA INC) describes a method of processing data that includes receiving a plurality of binary electronic signals and generating an optical signal by a number of lasers that is equal to or greater than the number of binary electronic signals. The optical signal is generated at one of a plurality of intensity levels, and each intensity level represents a particular combination of bit values for the plurality of binary electronic signals. The optical signal is converted into an electronic signal having the plurality of intensity levels. An apparatus for processing data is provided that includes a plurality of lasers configured to emit light at a plurality of frequencies, and a plurality of modulators configured to receive a plurality of binary electronic signals and to modulate the light